MRI (magnetic resonance imaging) is a kind of medical equipment that has been used since the beginning of the 1980s. Doctors use it to examine and diagnose quite a number of illnesses and disorders. The MRI scan uses magnetic and radio waves, meaning that there is no exposure to X-rays or any other damaging forms of radiation.

How does an MRI scanner work?

The patient lies inside a large, cylinder-shaped magnet. Radio waves 10,000 to 30,000 times stronger than the magnetic field of the earth are then sent through the body. This affects the body's atoms, forcing the nuclei into a different position. As they move back into place they send out radio waves of their own. The scanner picks up these signals and a computer turns them into a picture. These pictures are based on the location and strength of the incoming signals.

Our body consists mainly of water, and water contains hydrogen atoms. For this reason, the nucleus of the hydrogen atom is often used to create an MRI scan in the manner described above.

Using an MRI scanner, it is possible to make pictures of almost all the tissue in the body. The tissue that has the least hydrogen atoms (such as bones) turns out dark, while the tissue that has many hydrogen atoms (such as fatty tissue) looks much brighter. By changing the timing of the radiowave pulses it is possible to gain information about the different types of tissues that are present.

An MRI scan is also able to provide clear pictures of parts of the body that are surrounded by bone tissue, so the technique is useful when examining the brain and spinal cord.

Because the MRI scan gives very detailed pictures it is the best technique when it comes to finding tumours (benign or malignant abnormal growths) in the brain. If a tumour is present the scan can also be used to find out if it has spread into nearby brain tissue.

The technique also allows us to focus on other details in the brain. For example, it makes it possible to see the strands of abnormal tissue that occur if someone has multiple sclerosis and it is possible to see changes occurring when there is bleeding in the brain, or find out if the brain tissue has suffered lack of oxygen after a stroke.

The MRI scan is also able to show both the heart and the large blood vessels in the surrounding tissue. This makes it possible to detect heart defects that have been building up since birth, as well as changes in the thickness of the muscles around the heart following a heart attack. The method can also be used to examine the joints, spine and sometimes the soft parts of your body such as the liver, kidneys and spleen.

Advantages & Disadvantages

MRI has several advantages such as

it acquires patient information without the use of ionizing radiation;

it produces excellent soft tissue contrast;

it can acquire images in the transverse (axial), sagittal, coronal, or oblique (orthogonal) planes; and

image quality is not affected by bone.

The disadvantages primarily associated with MRI would include:

any contraindication that would present a detrimental effect to the patient or health care personnel;

long scan time when compared to CT; and

cost

The effects of the magnetic field, varying gradient magnetic fields, or the RF energy used pose the greatest harmful effects to biomedical implants that may be in the patient's body. Before entrance into the strong magnetic field can be obtained, everyone including patients, family members, health care professionals, and maintenance workers must be screened for any contraindications that my result in injury to themselves or others.

These may include any biomedical implant or device that is electrically, magnetically, or mechanically activated such as pacemakers, cochlear implants, certain types of intracranial aneurysm clips and orbital metallic foreign bodies.

These may include any biomedical implant or device that is electrically, magnetically, or mechanically activated such as pacemakers, cochlear implants, certain types of intracranial aneurysm clips and orbital metallic foreign bodies.